Some background on ALD and CVD may assist the reader in understanding the present disclosure. ALD is generally considered to include processes in which precursors alternately react at an exposed surface in a self-limiting manner with no direct interaction except for the exposed surface. CVD is generally considered to include processes in which one or more precursors react above a surface to form a material that ultimately deposits over the surface in a continuous deposition or not self-limiting manner. However, the distinction between ALD and CVD processes often blurs in actual applications so that processes that primarily proceed by ALD may still have some CVD-type contribution.
Typically, ALD includes exposing an initial substrate to a first chemical species to accomplish chemisorption of the species onto the substrate. Theoretically, the chemisorption forms a monolayer that is uniformly one atom or molecule thick on the entire exposed initial substrate. In other words, a saturated monolayer. Practically, as further described below, chemisorption might not occur on all portions of the substrate. Nevertheless, such an imperfect monolayer is still a monolayer in the context of this document. In many applications, merely a substantially saturated monolayer may be suitable. A substantially saturated monolayer is one that will still yield a deposited layer exhibiting the quality and/or properties desired for such layer.
The first species is purged from over the substrate and a second chemical species is provided to chemisorb onto the first monolayer of the first species. The second species is then purged and the steps are repeated with exposure of the second species monolayer to the first species. In some cases, the two monolayers may be of the same species. Also, a third species or more may be successively chemisorbed and purged just as described for the first and second species. It is noted that one or more of the first, second and third species can be mixed with inert gas to speed up pressure saturation within a reaction chamber.
Purging may involve a variety of techniques including, but not limited to, contacting the substrate and/or monolayer with a carrier gas and/or lowering pressure to below the deposition pressure to reduce the concentration of a species contacting the substrate and/or chemisorbed species. Examples of carrier gases include N2, Ar, He, Ne, Kr, Xe, etc. Purging may instead include contacting the substrate and/or monolayer with any substance that allows chemisorption byproducts to desorb and reduces the concentration of a species preparatory to introducing another species. A suitable amount of purging can be determined experimentally as known to those skilled in the art. Purging time may be successively reduced to a purge time that yields an increase in film growth rate. The increase in film growth rate might be an indication of a change to a non-ALD process regime and may be used to establish a purge time limit.
ALD is often described as a self-limiting process, in that a finite number of sites exist on a substrate to which the first species may form chemical bonds. The second species might only bond to the first species and thus may also be self-limiting. Once all of the finite number of sites on a substrate are bonded with a first species, the first species will often not bond to other of the first species already bonded with the substrate. However, process conditions can be varied in ALD to promote such bonding and render ALD not self-limiting. Accordingly, ALD may also encompass a species forming other than one monolayer at a time by stacking of a species, forming a layer more than one atom or molecule thick. The various aspects of the present invention described herein are applicable to any circumstance where ALD may be desired. It is further noted that local chemical reactions can occur during ALD (for instance, an incoming reactant molecule can displace a molecule from an existing surface rather than forming a monolayer over the surface). To the extent that such chemical reactions occur, they are generally confined within the uppermost monolayer of a surface.
Traditional ALD can occur within frequently-used ranges of temperature and pressure and according to established purging criteria to achieve the desired formation of an overall ALD layer one monolayer at a time. Even so, ALD conditions can vary greatly depending on the particular precursors, layer composition, deposition equipment, and other factors according to criteria known by those skilled in the art. Maintaining the traditional conditions of temperature, pressure, and purging minimizes unwanted reactions that may impact monolayer formation and quality of the resulting overall ALD layer. Accordingly, operating outside the traditional temperature and pressure ranges may risk formation of defective monolayers.
The general technology of chemical vapor deposition (CVD) includes a variety of more specific processes, including, but not limited to, plasma enhanced CVD and others. CVD is commonly used to form non-selectively a complete, deposited material on a substrate. One characteristic of CVD is the simultaneous presence of multiple species in the deposition chamber that react to form the deposited material. Such condition is contrasted with the purging criteria for traditional ALD wherein a substrate is contacted with a single deposition species that chemisorbs to a substrate or previously deposited species. An ALD process regime may provide a simultaneously contacted plurality of species of a type or under conditions such that ALD chemisorption, rather than CVD reaction occurs. Instead of reacting together, the species may chemisorb to a substrate or previously deposited species, providing a surface onto which subsequent species may next chemisorb to form a complete layer of desired material.
Deposition methods, such as atomic layer deposition (ALD) and chemical vapor deposition (CVD) methods, are commonly utilized to form material during semiconductor device fabrication. The deposition methods can be conducted to process a single semiconductor wafer at a time, or to process multiple semiconductor wafer at one time. It can be desired to process batch of multiple semiconductor wafer in order to increase throughput of a fabrication process, however, such can also lead to difficulties in forming uniform thicknesses of material across the semiconductor wafer.
FIGS. 1 and 2 illustrate an exemplary deposition apparatus 10 which can be utilized for either an ALD process or a CVD process. Although FIGS. 1 and 2 are illustrating the same apparatus, the method of illustration is different so that the apparatus is shown in a more generalized manner in FIG. 2 than in FIG. 1. For instance, FIG. 1 shows that the apparatus comprises a cylindrical shape with a liner 17 contained within an outer wall 15; whereas FIG. 2 does not provide specific detail about the shape of the apparatus or the presence of the liner. Although FIG. 1 has more detail than FIG. 2, it is to be understood that FIG. 1 is still a generalized diagram and that other features can be included in addition to those shown in FIG. 1. For instance, heating coils can be included in the apparatus.
The apparatus 10 comprises a reaction chamber 12 having a plurality of semiconductor wafer substrates 14, 16, 18, 20 and 22 retained therein. Such substrates would be retained within a substrate support structure which is not shown in the diagrammatic views of FIGS. 1 and 2.
An inlet 24 and outlet 28 extend into the chamber 12. In operation, precursor is flowed into the reaction chamber through the inlet, and substances (for instance, reaction by-products, unreacted precursor, etc.) are exhausted out of the reaction chamber through the outlet. The precursor can be considered to flow in a stream 30 (such stream can also be referred to as a bulk flow of the precursor). Valves (not shown) can be provided to control flow of matter into and out of the reaction chamber. Also, there can be various pumps and other fluid-flow control devices (not shown) associated with apparatus 10 for assisting in flowing matter into and out of the reaction chamber.
In the exemplary apparatus of FIGS. 1 and 2, precursor flow into the chamber is directed upwardly through the chamber along lateral edges of the substrates 14, 16, 18, 20 and 22. The precursor then diffuses between the substrates (with such diffusion being diagrammatically illustrated as flow of stream 30 over and between the substrates in FIG. 2) and forms material (not shown in FIGS. 1 and 2) ultimately deposited over surfaces of the substrates 14, 16, 18, 20 and 22.
In CVD processes, it can be desired that a rate of reaction of precursor to form deposited material be relatively slow so that the material is relatively evenly distributed across surfaces of substrates 14, 16, 18, 20 and 22 as the precursor mixture diffuses over and between the substrates. In other words, it can be desired that a rate of reaction of precursor to form deposited material be optimal for the diffusion rate of the precursor over and between the semiconductor wafer substrates. If the rate of reaction is too fast, deposited material will build up much faster at the edges of the substrates than at central locations of the substrates.
Due to the desire to have relatively slow formation of deposited material, relatively slow reacting precursors will typically be utilized in batch CVD processes. As an example, a silicon nitride deposit can be formed utilizing dichlorosilane and ammonia as precursors, or can be formed utilizing silane and ammonia as precursors. If a single wafer is treated by a CVD process, it will frequently be silane and ammonia utilized as precursors, in that such are the fast-reacting precursors and accordingly utilization of such precursors can increase throughput. Alternatively, if a batch CVD process is utilized it will typically be dichlorosilane and ammonia utilized as precursors in order to enable a more uniform coating to be deposited across the multiple substrates of the batch than could be achieved with the faster reacting precursors.
Even if relatively slow reacting precursors are utilized, there can be problems with the uniformity of deposition achieved across a semiconductor wafer substrate surface during a batch CVD process. For instance, FIG. 3 shows the semiconductor wafer substrate 20 after a coating (in other words, deposited material) 50 has been formed across an upper surface of the substrate. The substrate 20 can be considered to comprise a central region 21, and an edge region 23 surrounding the central region. Dashed lines 25 are illustrated in FIG. 3 to diagrammatically represent boundaries between the central region 21 and the edge region 23. The coating 50 is shown to be formed significantly thicker in the edge region than in the central region. Such non-uniformity of deposition of coating 50 can significantly complicate further processing.
FIGS. 4-7 describe a prior art method which has been developed to address the problem illustrated in FIG. 3. Referring initially to FIG. 4, such shows an assembly 63 comprising a semiconductor wafer substrate 60 at a preliminary processing stage. As will be known to persons of ordinary skill in the art, a semiconductor wafer substrate can comprise any of numerous semiconductor materials, and frequently will comprise, consist essentially of, or consist of silicon. To aid in interpretation of the claims that follow, the terms “semiconductive substrate” and “semiconductor substrate” are defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.
The substrate 60 of assembly 63 is retained by support pins 61 joined to rails 59. The assembly 63 also includes a boat 62 (which can also be referred to as a ringboat) forming a physical extension proximate the substrate, and supported by the pins 61. The boat 62 will commonly comprise, consist essentially of, or consist of quartz. The boat 62 and substrate 60 can be considered to form a sub-assembly 58, which is shown in top view in FIG. 5.
A gap 65 is shown between the substrate 60 and boat 62 in the views of FIGS. 4 and 5 to assist the reader in distinguishing the boat from the substrate. In practice the boat can be provided very close to the substrate, or even in contact with an edge of substrate.
The top view of FIG. 5 shows that the semiconductor wafer substrate 60 can comprise a circular configuration, and that the boat 62 can be an annular ring provided around the substrate.
FIG. 6 shows the construction 58 after it has been subjected to a batch CVD process of the type described with reference to FIG. 3. Such has formed the deposit 50 across an upper surface of substrate 60, and across an upper surface of boat 62.
Referring next to FIG. 7, the boat 62 (FIG. 6) can be removed to leave the substrate having a relatively uniform layer of the deposit 50 thereon.
A problem with the processing of FIGS. 4-7 is that the boat is an added expense and has a limited lifetime.
Problems similar to those discussed above with reference to FIG. 3 occur in ALD processes. In theory, if processes were truly entirely ALD such problems should not occur in that deposition occurring during ALD should be self-limiting. Accordingly, deposits should form as a single monolayer across a substrate surface, and there should not be problems with build-up of deposit at edges of the substrate. However, ALD processes frequently have a CVD-type component, and such component can lead to the problematic edge build-up comparable to the build-up discussed above with reference to FIG. 3 for a CVD process.
Although the problems of FIG. 3 are discussed relative to batch processes, it is to be understood that similar problems can also occur in single wafer processes.
The problem of FIG. 3 is but one of several problems that can occur during utilization of a deposition apparatus, such as the exemplary apparatus of FIGS. 1 and 2. Another problem that can occur is that precursor can become depleted within the flow 30 as the flow migrates through the reaction chamber of the apparatus.
It would be desirable to develop methods and apparatuses which can alleviate, and preferably prevent, the edge build-up problems discussed above with reference to FIG. 3 for CVD and ALD processes. It would be further desirable to develop methods and apparatuses which can address some of the other problems associated with deposition processes, such as, for example, the problem of precursor depletion that can occur in batch processes.